When spent pulping liquor containing sodium or potassium salts is combusted in order to recover the heat content of the organic material dissolved during the cooking and the chemicals contained in the spent liquor, the alkali metals begin to vaporize to a gas phase when the temperature exceeds 1000.degree. C.; the smaller the air coefficient and the higher the temperature is, the intenser the vaporization becomes. By supplying the strongly substoichiometric combustion with additional heat energy by preheating with plasma, auxiliary fuel or other combustion gas, it is theoretically possible to reach conditions where e.g. the combustion of sulfate black liquor produces gas the temperature of which is over 1000.degree. C., even 1500.degree. C., and which contains CO, CO.sub.2, H.sub.2 O and H.sub.2 S and in which the alkali metals are substantially in the form of a monotomic gas. This is disclosed by the Swedish patent application No. 8302245.9.
The application also discloses a method of condensing the alkali salts to a melt or to an aqueous solution from the gas phase by cooling the gas produced by the combustion as described above whereby the cooking chemicals are directly recovered as NaOH and Na.sub.2 S.
In order to recover the cooking chemicals directly, it is necessary to cool the gas very quickly to prevent the CO.sub.2 contained in the gas from reacting with NaOH or Na.sub.2 O to form sodium carbonate. If sodium carbonate is formed, the advantage provided by the method, i.e. the direct production of cooking chemicals, e.g. NaOH, is lost.
In conventional cooling the NaOH/Na.sub.2 O/Na is condensed onto the cooling surface in the melt temperature range concerned and forms a layer the thickness of which increases until the surface temperature reaches the melting point. The condensed material is removed slowly from the cooling surfaces, providing the melt with a long contact time with the CO.sub.2 in the gas. The heat transfer is decreased. The melt layer is corroding and decreases the heat exchange capacity, it also shortens the endurance expectancy of the device. Finally, the gas is cooled slowly and the Na.sub.2 C.sub.3 content in the melt increases with the result that it is no longer an advantage to use the melt directly as a cooking chemical.